1. Field of the Invention
The present invention relates to a wireless communication device including a resonant circuit connected to a wireless IC device and a radiation conductor connected to the resonant circuit.
2. Description of the Related Art
Recently, radio frequency identification (RFID) systems have been used for various applications such as commodity management. An RFID system includes a reader/writer and an RFID tag. To transmit information to each other through non-contact communication, a reader/writer and an RFID tag each include a wireless IC device (i.e., RFID IC chip) and a radiation conductor (i.e., antenna).
When information is to be transmitted, the wireless IC device modulates a carrier by using information that is to be transmitted, generates a high frequency signal, and outputs the generated high frequency signal to the radiation conductor. The radiation conductor transmits (i.e., radiates) the received high frequency signal to a communication target. When information is to be received, the radiation conductor receives a high frequency signal from a communication target, and outputs it to the wireless IC device. The wireless IC device reproduces information from the received high frequency signal.
In known techniques, examples of the RFID tag described above include wireless communication devices described in Japanese Patent No. 4301346 and Japanese Patent No. 4535209. In Japanese Patent No. 4301346 and Japanese Patent No. 4535209, a wireless communication device includes an electromagnetic coupling module and a printed wiring circuit board on which a loop-shaped electrode is formed. The electromagnetic coupling module includes a wireless IC device and a feeder circuit substrate on which the wireless IC device is mounted. The feeder circuit substrate is a multilayer substrate having a plurality of dielectric layers stacked one on top of another. Coil patterns constituting an inductance element and electrode patterns constituting a capacitance element are formed in the multilayer substrate. The inductance element and the capacitance element form a resonant circuit having a resonant frequency corresponding to a carrier frequency. The electromagnetic coupling module is mounted on the printed wiring circuit board, and is electrically connected to the loop-shaped electrode.
However, in the above-described wireless communication device, the coil patterns (pattern conductors) are formed in the multilayer substrate, resulting in a first problem in that loss caused by a material of the pattern conductors or dielectric substrates is apt to increase. Especially when the size of the multilayer substrate is to be reduced, it is necessary, for example, to narrow the line width of a coil pattern in order to obtain a desired inductance value, causing the insertion loss to be further increased.
The above-described wireless communication device has a second problem in that misalignment of a dielectric layer causes variations in the capacitance of the capacitance element in the multilayer substrate.